Field of the Disclosure
The present disclosure generally relates to a long-stroke pumping unit. The present disclosure also relates to a dynamic control system for a long-stroke pumping unit.
Description of the Related Art
To obtain hydrocarbon fluids, a wellbore is drilled into the earth to intersect a productive formation. Upon reaching the productive formation, an artificial lift system is often necessary to carry production fluid (e.g., hydrocarbon fluid) from the productive formation to a wellhead located at a surface of the earth. A sucker rod lifting system is a common type of artificial lift system.
The sucker rod lifting system generally includes a surface drive mechanism, a sucker rod string, and a downhole pump. Fluid is brought to the surface of the wellbore by reciprocating pumping action of the drive mechanism attached to the rod string. Reciprocating pumping action moves a traveling valve on the pump, loading it on the downstroke of the rod string and lifting fluid to the surface on the upstroke of the rod string. A standing valve is typically located at the bottom of a barrel of the pump which prevents fluid from flowing back into the well formation after the pump barrel is filled and during the downstroke of the rod string. The rod string provides the mechanical link of the drive mechanism at the surface to the pump downhole.
On any sucker rod lifting system, the dynamics of the rod string and the operation of the drive mechanism must be matched in order to prolong the service life of the lifting system. Conventionally, the combination of the output of a load cell connected to the rod string and software is used to determine certain operational characteristics of the rod dynamics and the downhole pump system. The operation of the surface drive mechanism is then controlled to achieve an optimum efficiency. This is a control philosophy that is limited in scope because the geometry of the drive mechanism is assumed to follow conventional pump-jack unit designs and certain rod dynamics are assumed based on historical values. This control philosophy is ill-suited for application to long-stroke pumping units because the operational geometry of the unit is different, particularly for the case of hydraulic pump-jacks where the geometry is pure reciprocation.
Also, long-stroke pumping units generally include a rotary motor, a gear box reducer driven by the motor, a chain and carriage linking the reducer to a counterweight assembly, and a belt connecting the counterweight assembly to the rod string. This type of drive mechanism is not very responsive to speed changes of the rod string. Gear-driven pumping units possess inertia from previous motion so that it is difficult to stop the units or change the direction of rotation of the units quickly. Therefore, jarring (and resultant breaking/stretching) of the rod string results upon the turnaround unless the speed of the rod string during the upstroke and downstroke is greatly decreased at the end of the upstroke and downstroke, respectively. Decreasing of the speed of the rod string for such a great distance of the upstroke and downstroke decreases the speed of fluid pumping, thus increasing the cost of the well.
Should the sucker rod string fail, there is a potential that the counterweight assembly will free fall and damage various parts of the pumping unit as it crashes under the force of gravity. The sudden acceleration of the counterweight assembly may not be controllable using the existing long-stroke pumping unit.